1. Field of the Invention
The present invention relates to an improvement in incoming-call transfer control for enhancing an incoming-call response rate concerning a call forward busy function of a private branch exchange system.
2. Description of the Related Art
As is well known, additional service functions of a private branch exchange (PBX) include a call forward busy function. The call forward busy function is, when an incoming call arrives at busy one of terminals belonging to the PBX, to forward the incoming call to a terminal (forwarding destination terminal) which is previously set for the above busy terminal.
The brief operation of the call forward busy function of this sort of prior art PBX will be explained by referring to a flowchart of FIG. 6.
Assume, for example, that, when one of the terminals belonging to the PBX is in a mutual talk with a party through a central office line or an extension line, an operator at the talking terminal receives a request for replacing the current talk by another extension line terminal. In this case, the operator first puts the central office line or extension line being now connected for the current talk temporarily in its hold state and then conducts a dialing operation to call the other extension line terminal to be newly connected thereto as an incoming-call destination terminal (step 601). After completion of the dialing operation, a central control circuit provided within the PBX judges whether or not an extension line connected to the incoming-call destination terminal is in use or busy (step 602). The central control circuit, when deciding that the incoming-call destination terminal is not busy, sends a call signal (bell ringing signal) onto this destination terminal (step 603) and also sends a ring-back tone to the terminal of the operator. The central control circuit, when the incoming-call destination terminal answers to the ringing, connects the incoming-call destination terminal to the central office line or the extension line being temporarily held, whereby the speech communication is established with the incoming-call destination terminal.
On the other hand, when the central control circuit determines in the step 602 that the incoming-call destination terminal is busy, the control circuit judges whether or not a terminal to which the call is to be transferred when the incoming-call destination terminal is busy (forwarding destination terminal) is set for the incoming-call destination terminal (step 604). The central control circuit, when determining such a forwarding destination terminal is not set for the incoming-call destination terminal, sends a busy tone to the operator terminal (step 605). At this time, the operator terminal, when listening to the busy tone, knows that it is impossible to connect the own terminal to the incoming-call destination terminal and must give up the interconnection therebetween or select another terminal as an incoming-call destination terminal. The central control circuit, when determining in step 604 that the forwarding destination terminal is set for the incoming-call destination terminal, retrieves its set data to find a telephone number corresponding to the forwarding destination terminal (step 606). In the illustrated example, the telephone number of the forwarding destination terminal is previously stored in a memory of the PBX, for example, in such a format as shown in FIG. 7. In the case of FIG. 7, when an incoming-call destination terminal A is busy, a forwarding destination terminal B is called. The central control circuit, when determining in the step 604 that the forwarding destination terminal is set for the incoming-call destination terminal, reads out the telephone number of the forwarding destination terminal from the memory (step 606). Thereafter, the central control circuit sends a call signal to the forwarding destination terminal on the basis of the read-out telephone number (step 607). The central control circuit then judges whether or not the forwarding destination terminal has answered to the ringing (step 608). If the forwarding destination terminal answers to the ringing, then the central control circuit connects the forwarding destination terminal to the central office line or extension line being held to establish a speech communication with the forwarding destination terminal.
As a method for previously storing the telephone number of the forwarding destination terminal in the memory, various methods may be considered including a method for storing it through a dialing operation of a special predetermined number and a telephone number for a forwarding destination terminal at the side of a terminal which intends to set the call forward busy function and a method for storing it through depression of a special key exclusively set in place of the above dialing operation of the special number and through a dialing operation of a telephone number of a forwarding destination terminal.
According to the conventional call forward busy function of the PBX, when an incoming-call destination terminal is busy, its incoming signal is forwarded immediately to a forwarding destination terminal. For this reason, the busy incoming-call destination terminal cannot know the presence of the incoming signal and thus cannot answer to it at all. This means that, according to the conventional call forward busy function, the incoming-call destination terminal in its busy mode, when receiving an additional new incoming signal, cannot respond to it and only its forwarding destination terminal can answer to the new incoming signal.
However, even the forwarding destination terminal has his own convenience and cannot always respond immediately to the forwarded incoming signal.
When the operator of the forwarding destination terminal is absent for example, the terminal cannot answer to it permanently. In such a case, the original caller must be kept waiting for a long time while remaining in the call state.
In this way, the call forward busy function of the prior art private branch exchange system has such a problem that, even when an additional incoming signal is directed to an incoming-call destination terminal in a busy state, the presence of the additional incoming signal is not informed to the incoming-call destination terminal and the additional incoming signal is immediately forwarded to a forwarding destination terminal preset for the incoming-call destination terminal, so that not only can the busy incoming-call destination terminal not know the presence of the additional incoming signal nor respond thereto but also the absence of an operator of the forwarding destination terminal results in the original caller being kept waiting while remaining in the call state, because even the forwarding destination terminal to be intended to respond to the incoming signal has its own convenience to handle.